refactor(sidebar): remove pin/unpin and global hide, use per-page hide only

- Remove pin/unpin and auto-hide hover mechanism (isPinned, isHovering,
  hoverTimeout, showSidebar/hideSidebar, updateAutoHideState, etc.)
- Remove global show_folder_sidebar setting (SettingsManager,
  PageControls, recipes, backend default)
- Simplify sidebar visibility to a single per-page toggle:
  · Dedicated chevron-left button in header to hide sidebar
  · Edge indicator (chevron-right) to restore when hidden
  · No dropdown, no hover area, no pin button
- Add _migrateOldSettings() to convert old sidebarPinned and
  show_folder_sidebar states to per-page sidebarDisabled
- Fix sidebar flicker on page load: CSS defaults to off-screen,
  JS explicitly sets .visible or .hidden-by-setting
- Remove obsolete CSS classes: auto-hide, hover-active, collapsed
- Remove i18n keys: pinSidebar, unpinSidebar, moreOptions
- Update test mocks for the new initialize() interface

tests/frontend/components/pageControls.filtering.test.js

@@ -20,7 +20,7 @@ const downloadManagerMock = {
 
 const sidebarManagerMock = {
   setHostPageControls: vi.fn(),
-  setSidebarEnabled: vi.fn(async () => {
+  initialize: vi.fn(async () => {
     sidebarManagerMock.isInitialized = true;
   }),
   refresh: vi.fn(async () => {}),
@@ -75,9 +75,6 @@ beforeEach(() => {
   performModelUpdateCheckMock.mockResolvedValue({ status: 'success', displayName: 'LoRA', records: [] });
 
   sidebarManagerMock.isInitialized = false;
-  sidebarManagerMock.setSidebarEnabled.mockImplementation(async (enabled) => {
-    sidebarManagerMock.isInitialized = enabled;
-  });
 
   global.fetch = vi.fn().mockResolvedValue({
     ok: true,